A typical dispensing device for supplying a heated liquid, such as a hot melt adhesive, generally includes a heated dispenser body constructed from a heat transferable metal, such as aluminum, brass, or stainless steel, and typically is coupled to a manifold, or other heater block, adapted to heat a liquid. The dispenser body includes a liquid inlet in fluid communication with the manifold to receive the heated liquid, and further includes a valve element that opens and closes a liquid outlet in communication with the inlet for dispensing discrete amounts of the heated liquid.
The valve element is usually operated by pressurized air to dispense the heated liquid. As such, an air passage or chamber typically is located in an upper area of the dispenser body, i.e. at an end opposite the liquid outlet. This air passage usually contains an actuating element, e.g. a piston, connected to the valve element and may further cooperate with a spring return mechanism. Under sufficient air pressure, the piston and valve element may be moved in a direction away from the liquid outlet to discharge liquid. When the air pressure on one side of the piston is relieved, the spring return mechanism will automatically return the valve element to a normally closed position. Air pressure may also be used to close the valve element. The spring return mechanism may be used to adjust valve stroke such as by varying its compression, thereby varying the amount of air pressure required to open the valve. Adjustment of the spring compression will also adjust the biasing force used to close the valve.
Notably, the dispensing devices related to the present invention generally situate at least one seal around the valve between the liquid passage and the upper area in an attempt to prevent heated liquid from entering the area. The seal usually is a standard O-ring, or spring-energized cup seal, tightly fit around the valve for axial movement therewith along an inner surface of the dispenser body. This seal arrangement, commonly referred to as a sliding-fit condition, can reduce the lifespan of the seal due to the reciprocating axial movement along the inner surface of the dispenser body. Also, costly, critical-surface machining on the surface of the valve element is required to provide the tight fit for the seal. Furthermore, the sliding-fit condition has a tendency to permit unfavorable leakage between the seal interface and the valve element. This leakage can reduce the performance of the dispensing device, for example, by decreasing the lifespan of the actuating element.
Accordingly, there is a need for an improved device for dispensing heated liquids, such as hot melt adhesives, which eliminates or reduces hydraulic seal leakage and critical valve surface machining thereby reducing costs and extending the life of the dispensing device.